research papers

A zone-plate-based two- spectrometer for indirect X-ray absorption spectroscopy

ISSN 1600-5775 Florian Do¨ring,a* Marcel Risch,b Benedikt Ro¨sner,a Martin Beye,c Philipp Busse,b,c,d Katharina Kubicˇek,c Leif Glaser,c Piter S. Miedema,c Jakob Soltau,e Dirk Raiser,d Vitaliy A. Guzenko,a Lukas Szabadics,b Leif Kochanneck,b Max Baumung,b Jens Buck,c Christian Jooss,b Simone Techertc,e,d and Christian Davida Received 3 October 2018 Accepted 21 March 2019 aLaboratory for Micro- and Nanotechnology, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland, bInstitute of Materials Physics, Georg-August-Universita¨tGo¨ttingen, 37077 Go¨ttingen, Germany, cPhoton Science, Deutsches Elektronen- Synchrotron DESY, 22607 Hamburg, Germany, dMax Planck Institute for Biophysical Chemistry, 37077 Go¨ttingen, e Edited by S. Svensson, Uppsala University, Germany, and Institute of X-ray Physics, Georg-August-Universita¨tGo¨ttingen, 37077 Go¨ttingen, Germany. Sweden *Correspondence e-mail: [email protected]

Keywords: two-color X-ray absorption X-ray absorption spectroscopy (XAS) is a powerful element-specific technique spectroscopy; zone plates; soft X-rays. that allows the study of structural and chemical properties of matter. Often an indirect method is used to access the X-ray absorption (XA). This work Supporting information: this article has supporting information at journals.iucr.org/s demonstrates a new XAS implementation that is based on off-axis transmission Fresnel zone plates to obtain the XA spectrum of La0.6Sr0.4MnO3 by analysis of three emission lines simultaneously at the detector, namely the O 2p–1s,Mn3s– 2p and Mn 3d–2p transitions. This scheme allows the simultaneous measurement of an integrated total fluorescence yield and the partial fluorescence yields (PFY) of the Mn 3s–2p and Mn 3d–2p transitions when scanning the Mn L-edge. In addition to this, the reduction in O fluorescence provides another measure for absorption often referred to as the inverse partial fluorescence yield (IPFY). Among these different methods to measure XA, the Mn 3s PFY and IPFY deviate the least from the true XA spectra due to the negligible influence of selection rules on the decay channel. Other advantages of this new scheme are the potential to strongly increase the efficiency and throughput compared with similar measurements using conventional gratings and to increase the signal-to- noise of the XA spectra as compared with a photodiode. The ability to record undistorted bulk XA spectra at high flux is crucial for future in situ spectroscopy experiments on complex materials.

1. Introduction X-ray absorption spectroscopy (XAS) is a widely used spec- troscopic technique that has gained increasing interest in the last decades because of its capability to obtain electronic as well as spatial information of many different sample classes. In XAS, a core-level electron is excited to an unoccupied state by the absorption of an X-ray photon (Yano & Yachandra, 2009). Depending on the proximity of the incident photon energy to the absorption edge of the investigated material, XAS is subdivided into near-edge X-ray absorption fine structure (NEXAFS) and extended X-ray absorption fine structure (EXAFS). The former provides element-specific information about the electronic structure of a material whereas the latter provides insights on the local arrangement of the atoms (Sto¨hr, 1996; deGroot & Kotani, 2008; Wende, 2004; deGroot, 1994). Typically, XAS is measured either directly in transmission or via electron or fluorescence yield channels. Although transmission measurements provide a direct method of obtaining X-ray absorption spectra, this option is often not

1266 https://doi.org/10.1107/S1600577519003898 J. Synchrotron Rad. (2019). 26, 1266–1271 research papers feasible due to the low transmittance for bulk samples, espe- the perpendicular direction. The optical setup has already cially in the soft X-ray regime. As an alternative, measure- been used for the analysis of resonant inelastic X-ray scat- ments of the electron yield (EY) and fluorescence yield (FY) tering (RIXS) of solid as well as of liquid samples (Marschall, offer an alternative of measuring XAS data indirectly, based McNally et al., 2017; Do¨ring et al., 2018; Marschall, Yin et al., on the assumed proportionality of the absorption cross section 2017). The complete setup consists of two off-axis transmission and the emission of electrons or fluorescence photons Fresnel zone plates that collect the emitted fluorescence from (Jaklevic et al., 1977; Gudat & Kunz, 1972). When comparing the sample and disperse it across a region of the CCD detector EY with FY measurements, it is apparent that EY is very (Fig. 1). surface sensitive because of the low escape depth of electrons This two-color scheme allows for focusing on two emission in matter. FY can be more bulk sensitive depending on the energies at the same time so that, under excitation at the Mn photon energy, but can also be applied with high surface L-edge, two PFYs (Mn 3d to 2p transition and Mn 3s to 2p sensitivity by using grazing-incidence geometry. Regarding FY transition) and the IPFY (O 2p to 1s transition, resulting from measurements, one can distinguish between collecting all the non-resonant O K-edge emission) can be detected using both emitted fluorescence integrated over all energies, which is zone plates at the same time. By integrating over the disper- called total fluorescence yield (TFY), or selecting certain sive direction on the detector, even a large part of the TFY can emission energy ranges by measuring the partial fluorescence be obtained simultaneously. The possibility to measure in yield (PFY). For the latter, an energy-sensitive detection principle both PFYs, IPFY and TFY, with two different zone scheme must be used. In the case of measuring the Mn L-edge plates at the same time makes this scheme very efficient and of La0.6Sr0.4MnO3 (LSMO), which will be of interest here, allows high-throughput experiments. In addition, the ability to conventional PFY is dominated by Mn 3d to 2p transitions, on two different emitted energies can be very interesting whereas high-resolution PFY also separates the 3s to 2p for measurements of complex materials or diluted samples, transitions located between the oxygen 2p to 1s emission and especially for in situ measurements where ambient conditions the manganese 3d to 2p emission. In general though, PFY at are varied. the Mn L-edge requires enhanced energy resolution because of the relatively small energy separation between the different 2. Materials and methods fluorescence channels. Besides TFY and PFY, the measure- The new XAS analyzer scheme presented here depends ment of inverse partial fluorescence yield (IPFY) has gained strongly on the optical components, namely the off-axis zone some attention (Achkar et al., 2011). In this method, one plates (see Fig. 2). They were fabricated by spin-coating a emission energy is chosen for a detailed spectral analysis, hydrogen silsesquioxane film on a 250 nm-thick Si3N4 where the fluorescence is non-resonant with respect to the membrane and writing the zone plate structure by high-reso- X-ray absorption and stems ideally from another element in lution electron beam lithography (Vistec EBPG 5000Plus). the compound. Assuming a constant cross section in this Each off-axis zone plate had a size of 3 mm  3 mm, an channel across the energy range of interest, the reduction of outermost zone width of 35 nm which corresponds to 14285 fluorescence in this channel is a direct measure for the lines mmÀ1, and a height of around 400 nm. absorption at the element of interest. Therefore, this channel For the two-color experiments, two different zone plates also suffers much less from self-absorption effects, making were necessary for the corresponding energies of the it possible to gain an undistorted bulk X-ray absorption absorption edges of the investigated materials. Their para- spectrum. meters can be found in Table 1. In this publication, we introduce a new analyzer scheme for The zone plates were placed between sample and detector XAS that uses transmission off-axis Fresnel zone plates with the energy-dispersive direction horizontal. In contrast to (TZPs). These optics are designed in a way that combines conventional reflecting variable-line-spacing grating analyzers, energy dispersion in one direction with spatial resolution in zone plates can be mounted very close to the optical axis such that only negligible offset for the beam path has to be considered in the setup. Moreover, zone plates have the advantage of being insensitive to , which allowed us to align the analyzer zone plates within just a few minutes.

Figure 1 Sketch of the two-color XAS analyzer scheme. Two transmission off-axis Fresnel zone plates are used to collect the emitted X-ray signal from the sample and to disperse it along one axis of the detector, while maintaining Figure 2 spatial resolution along the other axis. This allows for mapping the energy SEM images showing parts of the off-axis zone plates. Left is a top view of spread of the incident X-ray beam perpendicular to the emitted the broader inner zone structures. The right image was taken under an fluorescence at the same time. angle of 30 showing the height of the zones.

J. Synchrotron Rad. (2019). 26, 1266–1271 Florian Do¨ring et al.  A zone-plate-based two-color spectrometer 1267 research papers

Table 1 tioners, six off-axis zone plates of which two are used in Parameters of the zone plates used at different energies (Bearden, 1967; parallel and an order sorting (OSA). This optics Krause & Oliver, 1979). chamber was attached to the ChemRIXS endstation (Yin et Zone plate ID O-TZP Mn-TZP al., 2017), which was installed at the P04 beamline at PETRA Transition O 2p–1s Mn 3d–2p III in Hamburg, Germany (Viefhaus et al., 2013). The Chem- Photon energy (eV) 524.9 637.4 RIXS endstation provides a motorized sample stage and Object distance (m) 0.18 0.24 allows investigations of liquid jets and for conducting in situ (m) 0.16 0.20 Image distance (m) 1.15 1.09 experiments of solids in a water vapor atmosphere. The Outermost zone width (nm) 35 35 spectrometer arm of this setup is equipped with an ANDOR Outer radius (mm) 5.24 5.46 iKon-M 934 CCD with 1024  1024 pixels with a size of Size (mm  mm) 3  33 3 13 mm  13 mm. The distance between the CCD and sample was 1.33 m and, due to the flexibility of the zone plate Depending on the photon energy and the zone plate period, chamber, the whole setup could be realized with a straight the zone plates were mounted with a well defined inclination vacuum pipe without any offsets or bellows. Additionally, two towards the optical axis. This special geometry was chosen in SXUV100 photodiodes (Opto Diode Corp) were mounted order to benefit from volume effects inside the zone structures to simultaneously record the conventional total fluorescence and thereby enhancing the efficiency of the analyzer optics yield (PD-TFY). The emission energy scale of both zone (Hambach et al., 2001). In order to achieve maximum effi- plates was calibrated by setting the center of gravity of each ciency, we exploited these volume effects and optimized the transition to the tabulated energy of that emission line (i.e. O fabrication process concerning structure aspect ratio, which 2p–1s,Mn3d–2p, etc.) (Thompson et al., 2009). The incident we pushed towards the current possible limits in nanofabri- energy scale was calibrated by correcting for the shift between cation (Ro¨sner et al., 2018). For an estimation of the optimized the XAS spectrum extracted from the zone plate data (i.e. the efficiency, numerical calculations using rigorous coupled wave ROIs giving the TFY, PFY and IPFY) and that of the simul- analysis were performed. In the case of the O zone plate, taneously recorded PD-TFY. which was used at around 525 eV, we estimated an efficiency Highly crystalline LSMO thin films were chosen to establish of up to 15%, whereas for the Mn zone plate at 637 eV we the new detection scheme as they are free of pin holes, calculated an efficiency of up to 14% (Maser & Schmahl, uniform in structure and composition, and extremely flat. 1992). Furthermore, the material is of high interest for fundamental The zone plates, with optimized tilt angles, were mounted studies of energy storage processes at ambient temperature on a setup of Smaract nanopositioners, which was imple- (Risch et al., 2014; Scholz et al., 2016). The LSMO films were mented in a small vacuum chamber with 153 mm diameter. prepared by ion-beam sputtering as reported previously

This modular optics chamber can be easily connected to (Scholz et al., 2017). Nb-doped SrTiO3 (STNO) 0.5 wt% existing sample chambers. Fig. 3 shows the inner components (CrysTec GmbH) was used as the substrate. The films were of the modular optics chamber, consisting of eight nanoposi- deposited at 750C in an oxygen atmosphere of 1.7 Â 10À4 mbar. To reduce the amount of preparation-induced defects, the prepared films were kept under preparation conditions for 30 min and slowly cooled to room temperature, including a resting point of 30 min at 500C. These parameters were selected to yield films of 80 nm thickness as reported previously (Scholz et al., 2016). Off-axis XRD, as shown in Fig. 4(a), demonstrated that the LSMO grew epitaxially on the STNO substrate, since only the reflections of the {001} family were detected. The LSMO films

Figure 3 of the modular optics chamber. Six off-axis zone plates (each specially designed for one particular energy) can be loaded in the chamber. Two of them collect the radiation from the sample and disperse Figure 4 it across the detector. Those can be moved via three axes of translation, (a) Out-of-plane XRD of the used LSMO film (red) and an STNO each. In the background is an OSA that blocks stray light. The whole substrate (black) demonstrating epitaxial growth. (b) AFM image of the setup is mounted on a rotatable axis. film demonstrating a very low roughness of the order of a unit-cell step.

1268 Florian Do¨ring et al.  A zone-plate-based two-color spectrometer J. Synchrotron Rad. (2019). 26, 1266–1271 research papers have the expected double peaks where one matches that of the emitted energy on the x axis with the incident photon energy ˚ STNO substrate [aSTNO =3.91A(Mitchell et al., 2000)] and on the y axis was overlaid. Opening up the vertical mono- the other peak at a higher angle was attributed to the slightly chromator exit slit increased the photon flux significantly and shorter pseudo-cubic lattice parameter of LSMO [aLSMO = allowed for measuring incident energies between 650 eVand 3.88 A˚ (Zemni et al., 2004)]. Moreover, the films produced 653 eV in a single image. The lack of monochromated were extremely flat as shown by atomic force microscopy excitation is actually advantageous for our new approach as, (AFM) in Fig. 4(b). The film had a decorated terraced surface due to the imaging capabilities of the zone plates, the that reflects the terrace structure of the STNO substrate. The experimental energy resolution in the direction of the incident terraces were about 250 nm wide and the step height is energy dispersion is essentially limited by the size, to comparable with the pseudo-cubic lattice parameter of LSMO. 14 meV, which is much smaller than the intrinsic lifetime

Overall, the physical properties of the produced films equal broadening of the core hole (0.4 eV at the Mn L3-edge and those in our previous studies where additional characteriza- 1.5 eV at the Mn L2-edge; Fuggle & Inglesfield, 1992). tion and a discussion can be found elsewhere (Scholz et al., However, the analysis procedure yielding XAS at this very 2016, 2017). high resolution is complex and will be discussed elsewhere. Instead, the additional data is used here to reduce the statis- tical noise of the XAS as discussed below. The possibility to 3. Results and discussion decide in the analysis whether energy resolution of the X-ray We excited an epitaxial LSMO thin film with synchrotron absorption spectra or signal-to-noise is of greater importance radiation at different soft X-ray energies in the ranges 520– for that very experiment is one of the unique features of the 580 eV and 630–665 eV spanning across the absorption edges zone plate analyzer scheme. Together with the high efficiency of O-K and Mn-L3,2, respectively. For maximal flux, the and the – for zone plate values – large collected solid angle, vertical monochromator exit slit was fully opened, which measurements had an integrated flux of 18.27 Â 105 counts sÀ1 resulted not only in a high number of incident photons but under the full Mn-L 1,2 and L emission lines centered at also in a variation of the incident energy along the vertical axis 642.1 eV (in the range of 3.2 eV incident energy, corre- of about 0.5% spread (up to 3.8 eV) over a line of ca. 800 mm. sponding to the energy range collected in a single image with An essential advantage of zone plate analyzers is their imaging the 3d-TZP) as compared with 0.91 Â 105 counts sÀ1 using a capabilities that allow for mapping the variable incident conventional grating and monochromated radiation (Figs. S3 energy from the sample to the one axis of the detector and and S4). The flux at the detector was thus up to a factor of 17 combining this information with the fluorescence information higher using our new approach (Table S1). which is perpendicular to it (Marschall, Yin et al., 2017; Two independent maps of incident and emitted energy as Strocov, 2010). This enabled us to adjust the beamline to one imaged by two different zone plates can be collected at the energy value and probe the XAS around this energy by detector at the same time [see Fig. 5(a)]. The image in Fig. 5(b) mapping the range of incident energies together with the was focused on the Mn 3d–2p transition, which sharply imaged emitted energies at the same time. Such an acquired signal for the corresponding emission line. To the right, the defocused one range of incident energies is shown in Fig. 5. Mn 3s–2p transition is also visible before the imaging area Fig. 5 shows a representative detector image, which results ends. In Fig. 5(c), the zone plate was focused on the O 2p–1s from applying the two-color analyzer setup after setting the transition. Additionally, the Mn 3s–2p as well as the Mn 3d–2p central beamline energy to 651.5 eV. For each of the imaged transitions were imaged but defocused. Comparing Figs. 5(b) transitions, an appropriate coordinate system combining the and 5(c), it can clearly be seen that the maximum of the defocused image is lower and it is much wider on the emission axis. Finally, Fig. 5 illustrates that the images of the emis- sion lines must be separated vertically and horizontally to avoid interference between the signals originating from different zone plates, particularly of the defocused parts. X-ray absorption spectra were obtained by summing up the counts within a region of interest (ROI) in the Figure 5 obtained images measured at (a) Maps of incident and emitted energies from an LSMO sample collected simultaneously using a manganese zone plate (Mn-TZP) and an oxygen zone plate (O-TZP) recorded with the central beamline energies between 520 eV and beamline energy set to 651.5 eV. (b) Evaluation of the signal obtained through the Mn zone plate. 580 eV for the O K-edge and 630 eV to For easier comparison, the upper spectrum was flipped in order to correspond to the emitted energy 665 eV for the Mn L3,2-edge, both in axis. (c) Contribution of the O zone plate, which was slightly sheared in order to compensate a increments of 0.5 eV steps. The ROIs misalignment that resulted in a lightly tilted setup. Colored boxes correspond to the ROI where the XAS data were obtained as described in the text and evaluated in Fig. 6. The map was smoothed by were chosen for a width of 0.5 eV Gauss filtering (7); the original camera image can be found in Fig. S1 of the supporting information. around the corresponding incident

J. Synchrotron Rad. (2019). 26, 1266–1271 Florian Do¨ring et al.  A zone-plate-based two-color spectrometer 1269 research papers

Table 2 Signal-to-noise ratios (S/N) for all the spectra recorded with the TZPs in comparison with those of a photodiode (Fig. 6). Detection method S/N

O K-edge TFY TZP 657 2p-PFY 611 TFY PD 354

Mn L-edge TFY TZP 48 3d-PFY 46 Figure 6 IPFY 37 (a) Indirect XAS data for excitation at the O K-edge and (b) at the Mn 3s-PFY 22 L3,2-edges obtained by analysis of the detector images employing the TFY PD 17 zone plate scheme and a conventional photodiode (violet). All spectra were vertically shifted for clarity. normalized to 1. The edge jump (before normalization) is energy and the FWHM along the emitted energy. The sum in proportional to the total absorption of the material, which the ROI is then plotted against the beamline energy producing gives us the signal. The noise values were calculated as the the XAS spectra in Fig. 6, where they are compared with the root-mean-square in a relevant area below the onset of the output of a photodiode. The signal of the photodiode was also resonant absorption, namely from 521.5 eV to 524.5 eV for the K L used to align the FY spectra along the incident energy by O -edge and from 632.5 eV to 636.0 eV for the Mn -edge. It is apparent that the signal-to-noise ratio for the zone plate matching the maxima of the L2-edges. Note that the color of the ROIs in Fig. 5 and the color of the spectra in Fig. 6(b) analyzers are about twice that of the classical photodiode match, yet Fig. 5 represents only one point in the indirect XAS when the TFY is compared and even higher for most of the data excited at the Mn L-edge. In the lower energy range, only partial yields. the core holes in oxygen can be excited, over which the O K- Finally, we would like to point out that this scheme could, edge was scanned [Fig. 6(a)]. This has two consequences, in future, also be used for investigations on later transition s p e.g. namely that the image of the Mn 3d zone plate was not metals regarding, for instance, the M 3 –2 emissions ( Fe s p d p e.g. d p analyzed for this edge and that the spectra obtained by the 3 –2 at 615.2 eV) or the M 3 –2 transitions ( Fe 3 –2 at TFY and PFY are nearly identical to that of the photodiode. 705.0 eV). With the possibility to store more than two zone On closer inspection, the pre-peak at 530 eV was reduced plates in the optics chamber, these transitions could be in the spectra of the photodiode, which might be avoidable by captured by changing to a specially made zone plate. Thus, we an optimized placement of the diode in the measurement recommend the two-color scheme with one zone plate focused on oxygen (O 2p–1s 524.5 eV) for IPFY and a second zone chamber. The XAS spectra at the Mn L3,2-edge differ signifi- cantly [Fig. 6(b)]. For these spectra, we used the images plate focused on the higher transition for PFY. of both zone plates, where the Mn 3d-PFY, the Mn 3s-PFY and the zone plate TFY was obtained on the Mn 3d zone plate 4. Conclusions while the IPFY was obtained from the O 2p zone plate. The Mn 3s-PFY can also be obtained using the O 2p zone plate, In this work, we have shown the first implementation of off- which gives a qualitatively similar spectrum (Fig. S2). Here, we axis transmission Fresnel zone plates for XAS measurements used the spectrum from the Mn 3d zone plate as it is more in in a two-color scheme. This scheme offers the advantages of focus. In the extracted XAS spectra, the signal of the Mn L3- measuring the 3d-PFY, 3s-PFY, the IPFY and an integrated edge was drastically reduced as compared with the Mn L2- fluorescence yield resembling the TFY at the same time and edge for the TFY, either accessed through the zone plates with high efficiency. Thus, it allows undistorted XAS data of (TFY-TZP), the photodiode (TFY-PD) or the Mn 3d-PFY due complex materials with high throughput to be obtained. to self-absorption effects (Eisebitt et al., 1993). Only the Mn 3s-PFY and IPFY spectra showed a qualitatively undistorted Acknowledgements spectrum where the Mn L -edge is larger than the Mn L -edge. 3 2 Parts of this research were carried out at P04, PETRA III at A comprehensive discussion and quantification of the distor- DESY, a member of the Helmholtz Association. We thank the tions are beyond the scope of this manuscript. Nonetheless, we beamline scientists at P04 for their support. can conclude that our new detection scheme can successfully measure undistorted Mn-L3,2 XAS in contrast to the conventional photodiode. Funding information Another way of comparing the zone plate analyser with the The following funding is acknowledged: This work received photodiode is concerning their respective signal-to-noise ratio, funding from the EU-H2020 Research and Innovation as shown in Table 2. For the calculation of these values, the Programme under the Marie Skłodowska-Curie grant agree- signal was globally set to the edge jump height, which was ment (grant Nos. 701647, 654360) NFFA-Europe and the

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Deutsche Forschungsgemeinschaft, SFB755 (Projects B03, Marschall, F., Yin, Z., Rehanek, J., Beye, M., Do¨ring, F., Kubicˇek, K., B10) and SFB 1073 (Projects C02, C05) and the Helmholtz Raiser, D., Veedu, S. T., Buck, J., Rothkirch, A., Ro¨sner, B., Association (HGF) (grant No. VH-NG-1105). The Max Guzenko, V. A., Viefhaus, J., David, C. & Techert, S. (2017). Sci. Rep. 7, 8849. Planck Society (MPG, up to 2017) and the HGF are thanked Maser, J. & Schmahl, G. (1992). Opt. Commun. 89, 355–362. for financial support. (grant Nos. 701647, 654360, VH-NG- Mitchell, R. H., Chakhmouradian, A. R. & Woodward, P. M. (2000). 1105). Phys. Chem. Miner. 27, 583–589. Risch, M., Stoerzinger, K. A., Maruyama, S., Hong, W. T., Takeuchi, I. & Shao-Horn, Y. (2014). J. Am. Chem. Soc. 136, 5229–5232. References Ro¨sner, B., Koch, F., Do¨ring, F., Bosgra, J., Guzenko, V. A., Kirk, E., Achkar, A. J., Regier, T. Z., Wadati, H., Kim, Y., Zhang, H. & Meyer, M., Ornelas, J. L., Fink, R. H., Stanescu, S., Swaraj, S., Hawthorn, D. G. (2011). Phys. Rev. B, 83, 081106. Belkhou, R., Watts, B., Raabe, J. & David, C. (2018). Microelectron. Bearden, J. A. (1967). Rev. Mod. Phys. 39, 78–124. Eng. 191, 91–96. deGroot, F. M. F. (1994). J. Electron Spectrosc. Relat. Phenom. 67, Scholz, J., Risch, M., Stoerzinger, K. A., Wartner, G., Shao-Horn, Y. & 529–622. Jooss, C. (2016). J. Phys. Chem. C, 120, 27746–27756. deGroot, F. M. F. & Kotani, A. (2008). Core Level Spectroscopy of Scholz, J., Risch, M., Wartner, G., Luderer, C., Roddatis, V. & Jooss, C. Solids. Boca Raton, FL: CRC Press. (2017). Catalysts, 7, 139. Do¨ring, F., Marschall, F., Yin, Z., Rosner, B., Beye, M., Miedema, P., Sto¨hr, J. (1996). NEXAFS Spectroscopy. New York: Springer. Kubicek, K., Glaser, L., Raiser, D., Soltau, J., Guzenko, V. A., Strocov, V. N. (2010). J. Synchrotron Rad. 17, 103–106. Viefhaus, J., Buck, J., Risch, M., Techert, S. & David, C. (2018). Thompson, A., Lindau, I., Attwood, D., Liu, Y., Gullikson, E., Microsc. Microanal. 24(Suppl.2), 182–183. Robinson, A., Kim, K.-J., Scofield, J., Kirz, J., Underwood J., Eisebitt, S., Bo¨ske, T., Rubensson J. -E. & Eberhardt W. (1993). Phys. Kortright, J., Williams G. & Winick, H. (2009). X-ray Data Booklet. Rev. B, 47, 14103. Lawrence Berkeley National Laboratory, CA, USA. Fuggle, J. C. & Inglesfield, J. E. (1992). Editors. Unoccupied Viefhaus, J., Scholz, F., Deinert, S., Glaser, L., Ilchen, M., Seltmann, J., Electronic States. Berlin, Heidelberg: Springer-Verlag. Walter, P. & Siewert, F. (2013). Nucl. Instrum. Methods Phys. Res. Gudat, W. & Kunz, C. (1972). Phys. Rev. Lett. 29, 169–172. A, 710, 151–154. Hambach, D., Schneider, G. & Gullikson, E. M. (2001). Opt. Lett. 26, Wende, H. (2004). Rep. Prog. Phys. 67, 2105–2181. 1200–1202. Yano, J. & Yachandra, V. K. (2009). Photosynth. Res. 102, 241– Jaklevic, J., Kirby, J. A., Klein, M. P., Robertson, A. S., Brown, G. S. & 254. Eisenberger, P. (1977). Solid State Commun. 23, 679–682. Yin, Z., Peters, H.-B., Hahn, U., Gonschior, J., Mierwaldt, D., Krause, M. O. & Oliver, J. H. (1979). J. Phys. Chem. Ref. Data, 8, 329– Rajkovic, I., Viefhaus, J., Jooss, C. & Techert, S. (2017). J. 338. Synchrotron Rad. 24, 302–306. Marschall, F., McNally, D., Guzenko, V. A., Ro¨sner, B., Dantz, M., Lu, Zemni, S., Dhahri, J., Cherif, K., Dhahri, J., Oummezzine, M., X., Nue, L., Strocov, V., Schmitt, T. & David, C. (2017). Opt. Ghedira, M. & Vincent, H. (2004). J. Solid State Chem. 177, 2387– Express, 25, 15624. 2393.

J. Synchrotron Rad. (2019). 26, 1266–1271 Florian Do¨ring et al.  A zone-plate-based two-color spectrometer 1271